This invention relates to semiconductor laser diode assemblies of the can-seal type having a laser diode hermetically sealed in a space defined between a metal can and a stem assembled therewith, and more particularly to such assemblies which are reduced in size for use in optical pickup devices such as CD players, LD players, and CD-ROM players.
Semiconductor laser diode assemblies conventionally used for optical pickup devices for CD players etc. are generally constructed as shown in FIG. 7. As in the figure, the laser diode assembly of this kind has a cap unit hermetically sealing a laser diode for protecting against the external ambient, moisture, etc. The laser diode assembly has a metal stem 21 having a heat sink 21a formed by, for example, cold forging integrally with the stem 21, and two through-holes 21b formed through the stem 21 as shown in FIG. 8. A laser diode 23 is mounted on the upper end of the heat sink 21a via a sub-mount 22. A common electrode 28 is welded to the underside of the stem 21 to provide electrical connection to the stem 21 and the heat-sink 21a. On the other hand, lead electrodes 26, are held by the stem 21 via an insulating material such as a low melting glass 29 in a manner of being insulated therefrom. The laser diode 23 has one electrode electrically connected via conductor wires, not shown, to the lead electrodes 26, and the other electrode formed in an underside thereof being electrically connected via the sub-mount 22 and the stem 21 to the common electrode 28. The laser diode 23 is hermetically sealed in a space defined by the stem 21 and the cap 24. To provide hermetical seal, the cap 24 is conventionally welded onto the surface of the stem 21.
The cap 24, on the other hand, has a window formed with a window glass 25 at a top wall thereof, for transmission of light emitted by the semiconductor laser diode 23. The window glass 25 is adhered to an inner surface of the cap 24 through a low melting glass as an adhesive, not shown, in an airtight manner. The cap 24 is attached onto the stem 21 by means of electric welding. In order to enhance weldability, a projection 24b is formed in a bottom end of the flange 24a, i.e., the bottom of the cap 24 to provide linear contact with the surface of the stem 21, as shown in FIG. 8. The contact between the stem 21 and the cap 24 via the projection 24b causes concentration of electric current passed therethrough, with placing in the electrodes 31, 32 ensuring a connection between the stem 21 and the cap 24.
For semiconductor laser diode assemblies as stated above, the stem 21 is usually formed integrally with the heat sink 21a through cold forging from a thick sheet material, requiring a large scale of a fabricating system. There is therefore a problem of incurring mounting-up of fabrication cost, and accordingly inexpensive supply of such laser diode assemblies is difficult.
Furthermore, there is a disadvantage that there are inevitably encountered variations in contact between the welding projection 24b and the surface of the stem 21, resulting in variations in concentration of electric current passed therethrough and hence nonuniformity of welding. Accordingly, it is difficult to provide uniformity of welding and hence sealability for semiconductor laser diodes.
To cope with this, it may be considered that the cap be press-fitted onto the stem to offer hermetic seal between the cap and the stem. However, such method has not yet been practiced, because if so done there is high possibility of incurring cracks or breaks in the window glass or the low melting adhesive glass, when a pressing force is applied onto the top face of the cap.
For carrying out welding between the cap and the stem, a flange must be formed in the lower end of the cap, and an allowance margin has to be given to the stem to cope with deviation in diametrical position of the cap relative to the stem. However, such structure hinders compactness for semiconductor laser diode assemblies, particularly in diametric sizes.
In particular, there is a recent demand of reducing the overall or diametrical size for semiconductor laser diode assemblies, e.g., 3 mm or smaller in applications such as for CD players, LD players, or CD-ROM players. However, in the conventional structure, there is a necessity of providing an allowance margin of at least 0.5 mm to a stem and a flange in a cap of not smaller than 0.4 mm. To this end, there is a limitation for reducing the diametrical size of the package, to approximately 5.6 mm or smaller, thus making difficult to miniaturize semiconductor laser diode assemblies to a desired size.
Meanwhile, where a semiconductor laser diode assembly is employed in an optical pickup device including a diffraction grating, a laser diode assembly is entirely built into a holder 33, as shown in FIG. 9, by fixing the stem 21 onto the bottom of the holder 33. In such cases, a diffraction grating 32 is attached to an aperture formed in the holder 33 by being pressed against the inner surface of the holder 33 by the force of the spring 31. The spring 31 is held at the opposite end by a top face of the cap 24. However, the force of the spring 31 is usually set to approximately 3 to 5 kg for securing the difraction grating 32, which force is liable to cause cracks or breaks in a window glass or an adhesive glass fixing to the cap 24, hindering the sealability of the laser diode assembly.